I haven't used OS9 for a while but let me give it a try:
Go to the Apple Menu (upper left corner) and select "Control Pannels " and then "Monitors". Check in the "Resolution" tab if you can change the refresh rate. If you can, set it up to your desire rate.
Now, remember that OS9 is not plugandplay, which means that you could need a driver for your monitor. OSX will try to configure itself to the new monitor.
Hope this helps.

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